1. Field
The present disclosure relates generally to methods of selectively separating organic compounds from an aqueous mixture using membranes derived from copolymer compositions; more specifically it relates to methods of selectively separating one or more alcohols and/or one or more organic compounds from an aqueous mixture using membranes derived from polystyrene-polydialkylsiloxane-polystyrene triblock copolymers.
2. Related Art
Increasing concerns about global warming and decreasing amount of “easily” accessible oil reserves boosted the interest in biofuels in the last decade. The production of biofuels from renewable resources such as lignocellulosic feedstocks would allow production of fuel with no net carbon dioxide release to the atmosphere, therefore making biofuels an environmentally benign energy source. Biofuel production from lignocellulosic feedstocks consists of degradation of feedstock to fermentable sugars, fermentation of the sugars, and separation of alcohol from the fermentation broth. Conventionally, a distillation process separates the alcohol from the fermentation broth at the end of the fermentation process, but requires intensive energy resources and also suffers from azeotrope formation. Pervaporation separates biofuels from dilute aqueous solutions as an alternative technique to distillation. Since the alcohol concentration in fermentation broth is typically low (<10%), pervaporation is more economical and practical to separate the alcohol from the other components of the fermentation broth (water, sugar, bacteria and others).
Pervaporation is a membrane separation technique that is utilized to separate liquid mixtures through a membrane via a solution-diffusion mechanism. First, permeation through the membrane takes place and then the permeate is collected as a vapor on the other side of the membrane. The evaporation of the permeate on the permeate side of the membrane creates the driving force for the transfer of the permeate. The pervaporation membrane behaves as a selective barrier between the feed and the permeate; therefore, the selection of the pervaporation membrane is crucial to achieve high selectivity and fluxes. The permeability of the components through the membrane is the multiplication of their diffusion and solubility in the membrane material. For instance, for pervaporation of alcohol-water mixtures, the diffusivity of water is greater than the diffusivity of the alcohol due to the smaller dimension of the water molecule. Therefore, a membrane material with higher alcohol solubility should be selected to obtain high alcohol permselectivity.
Polydimethylsiloxane (PDMS) is well known as a membrane material for ethanol separation from dilute aqueous ethanol mixture due to its hydrophobic nature and high free volume which allows excellent selectivity and high fluxes. However, the low glass transition temperature of the polymer results in poor film-forming properties. Copolymers attracted significant attention because they can combine a variety of functional constituents into one molecule. For instance, one of the components can be chosen to promote ethanol transport while the other component provides the membrane with structural integrity. Therefore, the film forming properties of PDMS was improved by synthesis of PDMS containing copolymers for pervaporation experiments. A variety of PDMS containing graft and block copolymers containing polymethylmethacrylate (PMMA), polyalkylmethacrylates, polysulfone, polyurethaneurea, and polyimide have been synthesized and their pervaporation characteristics have been studied. Generally, mechanical properties of the block and graft PDMS copolymers as a function of temperature or molecular weight have not been quantified, and some block and graft PDMS copolymers possess poor mechanical properties unsuitable for pervaporation applications. Thus, there is a need for polymers that are highly selective for components of interest with mechanical properties suitable for membrane fabrication and/or pervaporation applications.